sarcoids in horses

Sarcoids are believed to be the most common skin tumor of the horse and affect about 2% of the worldwide population. Although they do not metastasize (spread to distant locations), they undoubtedly cause welfare concerns, especially in the ulcerated “fibroblastic” form. This may be through discomfort from the lesions themselves, from fly interference with the lesions, and also from administered treatments. In their most extreme forms, they can affect eyelid function and lead to secondary ulceration of the eye’s surface (Figure 1).

Clearly, they are not a “benign” lesion, despite their benign classification, and they should never be ignored. The presence of sarcoids also has financial implications not only due to the (sometimes very high) cost of treatment, but also due to reduced resale value.

There is compelling evidence that sarcoids are caused by a bovine papillomavirus, which is believed to be transmitted by flies, most likely from infected cattle, but possibly also from infected horses. It remains unclear exactly how the virus leads to neoplastic (cancerous) change, or why the virus is able to cause disease in more than one species. Interestingly, a (human) papillomavirus also is responsible for the vast majority of cases of cervical cancer and an increasingly large proportion of tumors of the head and neck in humans; clearly there is much to be learned about the implications of infection with papillomaviruses in all species.

There is little doubt that there is a heritable component to the disease. In Warmbloods, the heritability has been well demonstrated: Breeding two sarcoid-affected horses vastly increases the risk of producing a horse that develops sarcoids at some stage in its life. There also appear to be breed-related differences in sarcoid risk: Thoroughbreds are more likely to develop sarcoids than Standardbreds, and Lipizzaners seem more resistant to sarcoid development.

Unlike melanomas, there is no color predisposition and gender doesn’t affect the chance of developing the disease. Specific gene variations, noted in horses with sarcoids, are also associated with virally induced cancers in humans. The reasons for this association are unclear, but it is likely due to differences in immune function and may explain the apparent breed-specific variation in sarcoid risk.

sarcoids in horses

Treatment options for sarcoids are numerous, with no one option being suitable for every case. Traditionally, sarcoids were often left without treatment, but as they almost invariably become larger and more difficult to treat, early intervention is strongly recommended to avoid long-term sequelae. Treatment may involve topical or intra-lesional (administered into the lesion) chemotherapy or immune modulators, surgical procedures, or radiotherapy. Electrochemotherapy is a relatively new alternative treatment option with apparently very good results, but it unfortunately necessitates the use of multiple general anesthetics, so the risks must be carefully considered.

Appropriate treatment selection is largely dictated by the location and type of the lesion (Figure 2). Laser surgical excision is the author’s treatment of choice for the majority of lesions in locations other than around the eye; periocular lesions are treated via radiotherapy with great success. Reported success rates for laser surgical excision are in the order of 83% and for radiotherapy are around 90%; compared to other treatment options they represent the least risk to the horse and lead to the best results. However, not every lesion—or horse—is a suitable candidate for these treatments, and the key is to select the method most likely to be successful in the first instance.

Recurrent sarcoids become increasingly difficult to manage and convey a far worse prognosis than those appropriately treated in the first place.


CONTACT—Anna Hollis, BVetMed, Dipl. ACVIM, ECEIM, MRCVS——Animal Health Trust, Newmarket, U.K.

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.